Bornean orangutan description

The Bornean orangutan (Pongo pygmaeus) is one of only two species of orangutan, which together are the only species of great apes to be found in Asia (2). Orangutan means 'person of the forest' in the native languages of Indonesia and Malaysia, and orangutans are the largest tree-dwelling mammals in the world (1)(2).

Orangutans have a distinctive body shape, with very long arms that may reach up to two metres in length. They have a coarse, shaggy, reddish coat and grasping hands and feet (2). The skin of the face is bare and black, but can be pinkish around the eyes and muzzle in younger individuals (2). Orangutans are highly sexually dimorphic, with adult males being distinguished from females by their larger size (2).

The adult male Bornean orangutan occurs in two forms, flanged or unflanged (1)(4). Flanged males are larger than unflanged males, and also differ in possessing fleshy, protruding ‘flanges’, or cheek pads, on either side of the face. Flanged males also produce long vocalisations in order to attract receptive females (4).

The Bornean orangutan and Sumatran orangutan (Pongo abelii) were once considered to be subspecies of the same species. However, genetic evidence has since shown them to be different enough to be classified as two separate species (5). The Bornean orangutan differs from the Sumatran orangutan in having a broader face and shorter beard, and is also slightly darker in colour (6).

Related species

Bornean orangutan biology

A predominantly solitary creature, the Bornean orangutan occupies large overlapping home ranges where it spends almost all of its time in the trees, clambering between branches or using its body weight to bend and sway trees (2). Each night, the Bornean orangutan builds a nest from bent branches on which to sleep, high up in the trees (2)(7). An extra layer is added to cover the individual if it is raining (2).

Orangutans are the slowest breeding of all mammal species, with an inter-birth interval of approximately eight years (2)(8). They are long-lived, and females tend not to reproduce until they reach 15 years of age (2). The female Bornean orangutan is thought to prefer to mate with a dominant, flanged male, and will seek one out (4). However, other male orangutans may force matings should they encounter a lone female (2)(4).

The female Bornean orangutan gives birth to a single young after a gestation period of 235 to 270 days (2). The infant spends its first two to three years being carried constantly, and will still remain close to the female for at least another three years (2)(8).

The orangutan’s diet includes over 400 types of food, with fruit such as wild figs (Ficus spp.) and durians (Durio spp.) forming around 60 percent of its diet (2)(8). Leaves, shoots and invertebrates are also taken, and the Bornean orangutan will occasionally feed on mineral-rich soils (2).

Bornean orangutan range

Fossil evidence suggests that orangutans were once widely distributed in Southeast Asia. However, the Bornean orangutan is today restricted solely to the island of Borneo, with the largest population located in Kalimantan, the Indonesian part of the island (7).

Three subspecies are currently recognised, and their ranges are as follows; P. p. pygmaeus ranges from northwest Kalimantan to Sarawak, P. p. wurmbi occurs on southwest Kalimantan, and P. p. morio can be found on northeast Kalimantan to Sabah (1)(8).

Bornean orangutan threats

In the past, orangutans were hunted relentlessly in the majority of their Southeast Asian range, with their large size and slow movements making them easy targets for hunters (9). However, the main threat to orangutans today is loss of habitat. In the past twenty years, 80 percent of orangutan habitat has been lost to illegal logging, gold mining and conversion to permanent agriculture, in particular oil palm plantations. Orangutans are extremely vulnerable to exploitation, largely as a result of their extremely long inter-birth interval (8).

Forest fires raged through much of Borneo in 1997 and 1998 and it is estimated that around one third of the island's orangutan population was lost at this time (9). Orangutans that wander into oil palm plantations and other human-inhabited areas may also be captured for the illegal pet trade (8).

Bornean orangutan conservation

The Bornean orangutan is protected by law in both the Malaysian and Indonesian areas of the island, and is listed on Appendix I of the Convention on International Trade in Endangered Species (CITES), which prohibits international trade (3)(7). While some populations do occur within protected parks, illegal logging even within protected areas remains a key threat to the survival of this species, and has increased with political instability in Indonesia (2).

Current conservation priorities for the Bornean orangutan include the long-term protection its forest habitat (6). In order to ensure their long term success, conservation projects need to involve local communities that occur in close proximity to orangutan habitat (6). Conservationists are also working to reduce conflict between people and orangutans, by devising practical solutions to prevent orangutans from raiding crops (7).

Captive Bornean orangutans, often rescued from the illegal wildlife trade, are being re-introduced into the wild in rehabilitation centres (7)(8).

To learn more about a Whitley Award-winning conservation project for this species, click here.

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